AD426/506在做固件升级中，升级失败后可以按原步骤重新做升级。
参考答案：对
无伐滤池停用后如何启动？
－Excuse me, does Mr Smith’s son live here?
－He _________ be here, but he has moved.[ ]
A. has to
B. used to
C. tried to
D. happened to
